In addition to minimum 1000 high and 625 wide, Amazon recommends a 1.6 ratio, which upon experimentation looked for what I was doing -- too tall. My first piece is one of humor, and my instincts suggested the average such book would have a wider cover. After browsing a bit today, I'm finding quite a number of books in my area with a ratio of 1.275.
It's a subtle difference, but people pick up on cues they aren't consciously aware of. When you're truly on your own, you gotta play every angle.